Process of bonding copper foil to foil containing superconductive layer such as niobium stannide



PROCESS OF BONDIN 0 IL T OIL CONTAINING NOV. 14, Q D NK PPEH FO ISUPERCONDUCTIVE LAY SUCH AS NIOBIUM STANNIDE Original ed May 5, 1963 2 KFIG.2 FIG.2A

ROLL FROM ROLL United States Patent 3,352,008 PRGCESS OF BONDlNG COPPERFOIL TO FOIL CONTAINKNG SUPERCONDUCTIVE LAYER SUCH AS NIOBIUM STANNIDEDaniel F. Fairbanks, Winchester, Mass., assignor, by

mesne assignments, to National Research Corporation, Cambridge, Mass., acorporation of Massachusetts, newly organized Original application May3, 1963, Ser. No. 277,899, now Patent No. 3,309,179, dated Mar. 14,1967. Divided and this application Jan. 20, 1966, Ser. No. 550,068

3 Claims. (Cl. 29599) ABSTRACT OF THE DISCLOSURE Process for bondingcopper foil to a foil superconductor' of the type having a surface layerof Nb Sn. The foils are brought together with a low melting solder inbetween and passed through spaced rolls.

This application is a division of SN. 277,899, filed May 3, 1963, nowPatent No. 3,309,179.

This invention relates to superconductors, and more particularly toalloys known as hard superconductors, which are used in the manufactureof solenoid coils and the like.

It has been found advantageous to coat superconductive wire with acoating of copper or other material of low thermal and electricalresistance before winding it into high field magnetic solenoids. Thepresence of the copper permits the wire to carry currents which would beexpected on the basis of tests on short samples of the wire. Without it,the currents are considerably less; e.g., one-half of short-sample testvalues. Probably, the effect of copper is mainly due to its lowelectrical resistivity and its consequent ability to pass current aroundsections of the wire which may temporarily go normal. The bypassing ofthe current in this manner minimizes local heat production in the coiland reduces Variations in the magnetic field. In addition the presenceof copper is helpful in speeding the removal of heat from the winding toits surroundings.

The present invention seeks to achieve the benefits of such coating onNb Sn superconducting wires and ribbons of the type disclosed in thecopending application of Allen Stautfer, S.N. 133,653, filed Aug. 24,1961, and in the copending application of Allen, Das and Stautfer, S.N.207,320, filed July 3, 1962. However, it should be understood that theinvention is also applicable to various other elongated superconductorsin the form of wire, ribbon, plate and other regular shapes, asdescribed, for instance, in the copending application of Saur, S.N.208,925, filed July 10, 1962, and in Aviation Week and Space TechnologyMagazine, Oct. 9, 1961, p. 84. In these references, an outer coating ofNb Sn is formed on a substrate of refractory metal, as a decompositioncoating in the last case and as a diffusion coating in the others. Theexposed position of the Nb Sn layer militates against the use ofconventional extrusion or drawing processes for cladding the Nb Sn withcopper; the brittle Nb Sn layer might be damaged in such processes. Asnoted in the above copending application of Saur, the coating maycomprise hard superconductor alloys other than Nb Sn, such as Nb Al, VGa and V Si. The present invention is also applicable to suchvariations.

It is an object of the present invention to provide a technique ofpreparing superconductors clad with a heat dissipating and electriccurrent-bypassing coating, without resort to excessive compression ofthe superconductor.

It is a more specific object of the invention to provide a ribbonsuperconductor comprising a niobium base with an Nb sn coating and anouter layer of copper or other high conductivity substitutes, bonded tothe outer surface of the Nb Sn via a thin bond metal layer which iscompatible with the Nb Sn layer.

These and other objects of the invention will in part be obvious andwill in part appear hereinafter.

The invention accordingly comprises the process involving the severalsteps and the relation and order of one or more of such steps withrespect to each of the others, and the resulting product, which areexemplified in the following detailed disclosure and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a graph illustrating the considerations which govern thechoice of methods of forming a heat and current dissipating layer;

FIG. 2 is a cross-section of a superconductive ribbon made according tothe present invention;

FIG. 2A is a variation of FIG. 2 wherein the current carrying capacityis increased by the use of multiple superconductive layers;

FIG. 3 is a schematic diagram of one embodiment of the cladding processof the invention showing the application of conventional elements ofapparatus;

FIG. 4 is a schematic diagram showing a variation of another part of theprocess of FIG. 3.

FIG. 5 is a schematic diagram showing a variation of another part of theprocess of FIG. 3.

The metal of choice for the heat and current dissipating layer iscopper. However, in accord with the present invention, because of theirexcellent conductivities at cryogenic temperatures, tungsten or cadmiummay be used in similar fashion. Other metals which may be used arealuminum, indium, silver, lead, tin and sodium. In each case the pureelement should be used. Generally, the superconductor will be used atliquid helium temperature where the electrical and thermal resistivitiesof the pure metals are lower than those of their respective alloys.

As noted above, conventional extrusion and cladding processes are notsuitable for applying the copper layer, due to the brittle nature of thesuperconducting layer. Electroplating avoids the danger of damaging thesuperconducting layer, but tends to contaminate the copper layer. Theeffect of contaminants is shown in the graph of FIG/1 wherein curve A isthe resistivity curve of pure copper and curves B, C, D, E indicate theefi'ect of contaminants and alloying elements.

According to the present invention a very thin layer of metallic solderis interleaved between the superconducting and copper layers and thecladding is accomplished by heating the flux to its melting point. Thesolder is selected as an alloy of low melting point and good electricalconductivity at liquid helium temperature. The material designated forthis purpose in the present invention is the eutectix mixture of tin andindium, a commercially available solder having a melting point of aboutC. Further, this material has been discovered to be superconductive atliquid helium temperatures and low external magnetic fields up to 2kilogauss, a factor which enhances its suitability for present purposes.

FIG. 2 shows a cross-section of ribbon assembly 20 after the coppercladding is completed in accordance with the new method described below.The copper layer is bonded to the Nb Sn layer via a thin layer oftinindium eutectic. It has been found by experiment-s that a eutecticcomposition of tin-indium alloy in contactwith the N-b Sn layer will notadversely affect its superconductivity. It is believed that the alloycan be varied to as much as from 15:85 to 95:5 of tin to indium withsimilar results. It is also believed that a similar range of tin-leadalloys can be used.'It should be noted that a eutectic composition oftin-indium offers :a lower melting point than the above-suggestedalternates and that it is desirable to work at the lowest possibletemperatures to avoid contaminating the Nb Sn while working inatmosphere. Whatever solder is used, the temperature must be kept lowenough to avoid excess formation of copper alloys which are poor heatand electrical conductors compared to copper. Where cadmium or tungstenare used in lieu of copper, the choice of solder is wider since thesemetals are less reactive with tin (in the solder or in the Nb Sn layer)than is copper.

FIG. .2A shows a variation of FIG. 2 wherein the originalsuperconductive ribbon has Nb Sn coatings on both sides and copper isclad on both sides in accord with the present invention.

Copper clad ribbon, shown in FIGS. 2 and 2A can be wound into a magnetwithout further treatment. However, it is preferred, in each instance,to first coat the entire ribbon with a conventional dielectricinsulation. When the magnet is put into a cryogenic bath the currentwill be carried entirely by the Nb Sn layers. If small sections of theNb Sn layer return temporarily to normal state conduction of electriccurrent, the copper layers in intimate contact therewith will allowcurrent to bypass these sections and will minimize resistance heatingsince copper has a very low resistivity at cryogenic temperatures and asubstantial cross-section in the arrangement shown. The resistanceheating which does occur is dissipated by the excellent thermalconductivity of the cop per. When a section of ribbon goes normal, thecopper dissipates the resistance heat in a manner tending to avoidextreme localization of temperature rise in the ribbon with consequentdestruction of the ribbon at the overheated locality. The copper orother metallic foil used should be soft, annealed metal with a thicknessof .0005 to .001 inch. For simplicity, only the method for cladding theribbon shown in FIG. 2 is described below.

FIG. 3 shows copper (Cu), indium-tin (In-Sn) and superconductor (NbSn/Nb) foils being assembled into a foil assembly 2, passed over a roll10, and then into a liquid bath 12 (heated by heater 14) over rolls 16and 18 and then out of the bath past a shield 26, over a roll 20 wherethe composite foil (now indicated by number 4) is sprayed with coolantfrom a nozzle 24 and then rolled up at 22.

FIG. 4 shows a variation of the FIG. 3 apparatus wherein, instead ofinterleaving solder between the copper and super-conductive foils, thesolder is transferred from a bath 30 over rolls 32 and 34 to a coatingzone 36 where it is contacted by a conventional ultrasonic solderingiron 38 and then transferred to the roll 10 for contact with thesuperconductor foil to form the foil assembly 2.

Referring now to FIG. there is shown a variation of some of theapparatus in FIG. 3. The foil assembly 2 is passed between a stainlesssteel shelf 118 and rollers 116, where it is heated by heater 118 to themelting point of the solder and then passed through driven rollers 120to a wind-up roll 122. The minimum spacing between rollers 120 is equalto the thickness of the foil assembly 2, less the average thickness ofthe flux layer, and the maximum spacing is the thickness of the foilassembly 2, plus about 10%. Thus, a gentle compression is applied to thefoil assembly 2 to spread the solder and drive out included gaseswithout damaging the Nb Sn layer. Water cooling jets 124 and shields126, similar to those of FIG. 3, are also provided.

Since certain changes may be made in the above process without departingfrom the scope of the invention herein involved, it is intended that allmatter contained in the above description, and shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. A process of cladding a first foil of soft metal to a second foilcontaining a surface layer of hard superconductive alloy, selected fromthe group consisting of Nb Sn, Nb Al, V Ga, V Si, without breaking upsaid surface layer or degrading the superconductivity thereof and withthe result that the second foil when wound into magnetic solenoids isenabled to carry currents superconductively at levels substantiallyequal to the short, sample foil performance of the second foil, theprocess comprising the steps of forming a foil assembly with said firstand second foils in face-to-face relation, the superconductive surfacelayer facing inwardly, with a bonding metal between said foils in thefoil assembly, passing the foil assembly between rollers and a heatedplate to melt the bonding metal and then passing the foil assemblybetween a pair of spaced rolls having a spacing at least equal to thecombined thickness of the first and second foils and no greater than thecombined thickness plus about 10% so that a gentle compression isapplied to the foil assembly to spread the bonding metal and drive outincluded gasses Without damaging the superconductive layer, and thencooling the foil assembly.

2. The process of claim 1 wherein said second foil contains a saidsurface layer on each face, and two of said first foils are providedagainst opposite faces of said second foil.

3. The process of claim 1 wherein the bonding metal is initially coatedon at least one of said first and second foils before assembling thefirst and second foils.

References Cited UNITED STATES PATENTS 132,338 10/18772 Warden 29502 X2,984,901 5/1961 Cunningham 29501 X 3,184,303 5/1965 Grobin. 3,218,69311/1965 Allen 29--l55.5

WILLIAM I. BROOKS, Primary Examiner.

1. A PROCESS OF CLADDING A FIRST FOIL OF SOFT METAL TO A SECOND FOILCONTAINING A SURFACE LAYER OF HARD SUPERCONDUCTIVE ALLOY, SELECTED FROMTHE GROUP CONSISTING OF